Level Up & OSR Enthusiast
Yeah, a sandbox with elements in each point of interest that can (but don't force) the PCs to others points of interest, with an overarching threat on the whole region, is a great campaign set up for me.I use an episodic sandbox with an overarching threat. I guess Buffy the Vampire Slayer and to some extent Babylon 5 were my inspirations for this approach. The PCs can do what they want, but somewhere in the distance an active BBEG force is on the move. I will roll for its successes and failures, but generally slanted towards success, where other factions would be more random. So it might happen that offstage NPC good guys succeed in a desperate holding action to hold the mountain pass vs the BBEG army, but the chances are the BBEG is only slowed, not defeated. Eventually the PCs may rise to challenge the BBEG, and succeed or fail - one of my best ever campaigns was set in the aftermath of a previous campaign where the BBEG won.
Edit: I guess what I do somewhat resembles (edit) Dungeon World 'with its Fronts/Threats system and Progress Clocks', but more simulationist, I use a Free Kriegsspiel* type approach where I'll get in the heads of the NPC factions, determine their resources, goals & attempts/efforts, and where there's conflict I'll roll to resolve success, usually with a d6. I may not roll if success is certain, eg my Black Sun BBEG faction had a 'ringer' in the form of Kainos, Demigod Son of Ares-Bane. He was around CR 28, and when he went up against various level 8 or so NPC heroes, I didn't bother rolling.
*Derogatorily referred to as Mother May I by the uncouth.